La Casa Mila, nicknamed La Pedrera ("The Quarry") for its rough-hewn stone facade, is the last major residential building Gaudí designed. Built between 1905 and 1910, it wraps around Passeig de Gràcia with rippling balconies and an organic roofline that looks like nothing else on the boulevard.
The rooftop is the highlight -- a landscape of surreal chimneys and ventilation towers, with views across Barcelona to the Sagrada Familia. Below that, the attic museum explains Gaudí's structural innovations using hanging chain models. On the top floor, a reconstructed Art Nouveau apartment shows period furnishings and design details that are genuinely stunning.
Queues can be long during peak season. If you'd rather skip the full tour, the first-floor exhibition hall hosts rotating shows by contemporary artists and is free to enter -- you still get a feel for the building's interior without the wait or the ticket price.